Generation gap A cluster of stars stolen by the Milky Way from a nearby galaxy has a unique composition that could reveal new insights into the formation of these objects, according to astronomers.

According to Dr Sarah Martell of the Australian Astronomical Observatory, Terzan 7 -- a globular cluster orbiting the Milky Way and located 76,000 light years away -- only has a single stellar generation, unlike other nearby clusters which contain two generations.

Globular clusters are tightly bound spheres of about a million stars, usually found in the halo surrounding a galaxy.

"From its orbit … it looks like it may have originally belonged to the Sagittarius Dwarf galaxy, but has been kidnapped and now is part of the Milky Way," says Martell.

"It's also younger than other globular clusters," she says. "Most in our galaxy are almost as old as the universe … 12 or 13 billion years, but Terzan 7 is about 8 billion years old."

Martell and colleagues discovered Terzan 7's unusual stellar composition, while using the Very Large Telescope in Chile to measure the chemical signature spectrum of the light coming from stars in the cluster.

"We take spectra of individual stars in the cluster looking at the carbon and nitrogen," says Martell.

Missing generation

They found the signatures for these elements were very different in Terzan 7 compared to that seen in normal Milky Way globular clusters.

About half the stars in other globular clusters orbiting the Milky Way have the same composition as stars in the surrounding galactic halo, with the rest having a different chemical composition.

"The cause is a subject of active ongoing research," says Martell.

"The thought is there are actually two generations of stars. The ones with normal [chemical] abundances are the first generation of stars. They eventually eject material back into their surroundings and that contributes to the second generation."

According to Martell, the question is, "Why does Terzan 7 only have a single generation of stars?"

"Is there something special about globular clusters [that form] in dwarf galaxies that gives them a different [star] formation history from globular clusters in big galaxies," asks Martell.

"If the chemistry's distinct for globular clusters that form in dwarf galaxies, then you would expect a certain fraction of the clusters belonging to the Milky Way, to have this unusual [single stellar generation] behaviour," says Martell.

Astronomers plan to examine other globular clusters in the Sagittarius Dwarf galaxy to see if they are of similar composition to Terzan 7.

Alternate hypothesis

Martell believes an alternate possibility is that the comparatively young age of Terzan 7 means a second newer generation of stars haven't had a chance to form.

"It could just be that … globular clusters that formed in the early universe are complicated and globular clusters that formed a few billion years later are simple," says Martell.

"It hasn't had the time or available gas to make a second generation of stars."